The present invention relates to a projection television apparatus, and more particularly, to a focus adjusting means for such a projection television apparatus.
In order to provide a large picture, many types of projection television apparatuses have been proposed. A projection television apparatus that comprises three monochromatic cathode-ray tubes, two dichroic mirrors and a lens, has been been previously proposed, for example in U.S. Pat. No. 4,151,554.
Its basic construction will be described below:
Three cathode-ray tubes which display red, green and blue images, respectively are placed in the same plane to form the letter "T", with their faceplates facing each other and forming three sides of a parallelepiped space. Two dichroic mirrors are positioned in the space and are crossed at right angles to each other. The lens is placed on the axis of the center cathode-ray tube. Red, green and blue images from three cathode-ray tubes are combined into the color image by the two dichroic mirrors, and the resulting color image is enlarged and projected onto the screen by a single lens.
It is necessary for the projected image of each color to be adjusted individually for focus ("focus" described here is not the focus of the electron gun beam in the cathode-ray tube but that of the projecting lens exclusively). The customary way is by sliding the lens along its optical axis in order to focus roughly, and sliding each cathode-ray tube along its optical axis in order to focus each color finely. In this case a mechanism for focus adjustment is necessary so that each cathode-ray tube can be moved along its optical axis when focus adjustment is carried out, and can be fixed after focussing is finished. But such a mechanism becomes a very large-scale device because cathode-ray tubes are heavy and big. This mechanism, additionally, has the problem that persons using it are in danger of receiving an electric shock, because the focus adjustment is carried out while the cathode-ray tubes are driven at a high voltage (about 30 kV in general).